This invention relates to the improvement of the mineralogical and chemical composition of naturally occurring and synthetic alumina process feedstocks. The invention is particularly suited to the enhancement of boehmitic bauxites used in the production of alumina and alumina chemicals, especially by the Bayer process.
Embodiments of the present invention have the common feature of heating of the alumina process feedstock to bring about thermal dehydration and removal of organic carbon or conversion of organic carbon to a form which is not extractable in the aqueous phase digestion of the alumina process feedstock. Additional steps may be employed as will be described below.
The dominant technology for the extraction of refined alumina from alumina process feedstocks is the Bayer process. In the Bayer process alumina is extracted from alumina process feedstock (most frequently in the form of bauxite) by contacting the milled alumina process feedstock with hot caustic solution, generally under pressure, to dissolve alumina therefrom. If the alumina process feedstock contains mainly gibbsite (a mineral form of alumina trihydrate), extraction of alumina from the bauxite may be conducted using a caustic solution at a temperature generally in the range 100 to 175xc2x0 C. If the alumina process feedstock contains mainly boehmite, or diaspore (mineral forms of alumina monohydrate) higher temperatures, in the order of 200 to 300xc2x0 C. are generally required. The higher temperature digestion is required in these cases because the monohydrate forms act to cause instability of caustic solutions containing the high levels of dissolved alumina desired for subsequent processing unless there is a high degree of elimination of these forms by digestion at temperatures where such liquors will be stable. High temperature digestion comes with significant equipment cost disadvantages, in a much larger liquor heating and flashing system (e.g. 11 stages compared with 3) and in more expensive materials and specifications for construction. For mixed trihydrate and monohydrate forms, as is the case for many naturally occurring bauxites, a double digestion process, in which residues from a lower temperature first stage digest are further digested in a higher temperature second stage digest, may be used.
After digestion the digestion solid residue/pregnant caustic liquor mixture is brought back to atmospheric pressure by flashing to boil off water. The solid residue (usually referred to as red mud) is separated from the pregnant, caustic aluminate bearing liquor, usually by a combination of settling or filtration and washing, with both pregnant liquor and wash liquor clarified through pressure filters. The clarified combined liquor is fed to a precipitation circuit where it is cooled and seeded with solid particles of alumina trihydrate to induce precipitation of solid alumina trihydrate from the liquor. The resulting precipitation slurry is separated into a spent liquor stream and solids streams graded by particle size, by settling, cycloning or filtration, or combination of these processes. Coarse solids represent product, and are washed and transferred to a calcination stage where they are calcined to produce alumina. Intermediate and fine solids are separately returned to the precipitation circuit, frequently after at least crude deliquoring, e.g. in cyclones or filters, for agglomeration and to provide seed.
The fine seed is normally washed prior to recycle to precipitation, either to remove solid phase oxalate precipitated with the alumina (which would interfere with the incorporation of the fine material into composite coarse particles in the precipitation process), or to remove organic compounds which would otherwise render the seed less active.
The spent liquor is returned to the digestion step, normally after some reconcentration by evaporation, where it is contacted with further milled alumina process feedstock.
The Bayer process has been used commercially for about 100 years and is well known to persons skilled in the art.
Alumina process feedstocks, particularly bauxites, include a range of impurities in addition to the hydrated forms of alumina. The main impurities are compounds of iron, titania and silica, which, while having various deleterious effects in the Bayer process, including on consumables such as flocculants, lime and caustic soda, and on scale formation and product quality, deport predominantly to the solid mud residue.
Despite its presence at only low levels in typical Bayer process feeds, extractable organic carbon (0.02% to 0.35%) is an impurity of major significance. Organic compounds, carbonates and oxalates derived from organic carbon in the feedstock have the capacity to accumulate in the circulating liquors, sequestering caustic soda which could otherwise have delivered alumina from digestion to precipitation, and therefore severely impacting on the productivity of the process. While carbonates and oxalates can be removed from the circuit by causticisation of various wash liquors or precipitates with lime, a reduction in the level of other organic carbon derivatives can only be achieved by either pressure oxidation (which comes with explosion hazards and generates large quantities of oxalate and carbonate which then must be removed), or bleeding off of caustic solutions, for either neutralisation and disposal (which is major economic burden through caustic make-up costs) or for concentration by evaporation followed by destruction by combustion (which has high energy and capital costs). Organic compounds also interfere with the precipitation process (by adsorption onto active sites on the seed, having a seed poisoning effect) and carry soda as a contaminant into the precipitated product. Oxalate derived from organic carbon is relatively insoluble, and can precipitate as sodium oxalate with the alumina trihydrate, interfering with product size, morphology and chemistry, and reducing resistance to particle attrition. Because these effects lead to the necessity to ensure that oxalate is not precipitated in the same precipitation tanks in which fine alumina is to be cemented into composite particles by the early portion of the precipitating alumina hydrate, and because oxalate stability above its solubility is a strong inverse function of liquor strength, the caustic strength available for carrying alumina is also limited in most alumina refineries by the input of oxalate precursors and oxalate generated by oxidation of other organics.
That is, organics in alumina process feeds are in large measure responsible for establishing the limits to productivity in the Bayer process, by setting the maximum level of soda in liquor, determining the extent to which this soda is sequestered from its useful purpose of delivering alumina, and acting as poisons for the precipitation process.
The impact of monohydrate alumina in alumina process feeds in driving the need for high temperature digestion has already been mentioned. Some other impacts of monohydrate alumina should also be mentioned. Digestion of alumina process feeds at high digestion temperatures results in side reactions (such as production of titania phases) which reduce digestion efficiency. For this reason lime addition is frequently made. The consumption rate of lime for this purpose and for causticisation and oxalate destruction is sufficient to justify the construction of dedicated lime kilns in many environments. Also, the digestion temperature is frequently limited by the pressures at which boilers can operate safely and effectively, which results in a greater limitation on liquor alumina concentration for high temperature digestion than for low temperature digestion, given the instability of high alumina concentration liquors in the presence of solid residues which still contain destabilising monohydrate alumina. Thus digestion of monohydrate alumina bearing alumina process feeds is naturally less productive than digestion of alumina bearing feeds with little or no monohydrate alumina. To make up for this shortcoming some alumina processing plants inject alumina bearing feeds having little or no monohydrate alumina into the cooling digestion liquors in the flashing vessels at temperatures and for contact times for which monohydrate alumina in high temperature digestion residues will not quickly cause liquor decomposition. This process is known as sweetening. The process adds significantly to processing complexity, requiring a separate milling and slurrying system for the injected feed having the low content of monohydrate alumina. Since important reactions which result in silica in feedstock forming solid sodium aluminosilicates (and therefore deporting to residues) cannot be completed at the times and temperatures of liquor/solids contact for the injected feedstock the sweetening process also elevates the level of dissolved silica in digestion liquors, causing elevated levels of silica subsequently precipitated with the alumina hydrate, and scaling problems in evaporation, alumina process feedstock slurrying, and liquor and slurry heating. To prevent scaling problems an aluminosilicate seeded desilication operation after hydrate precipitation may be added to the flowsheet.
Further, high temperature digestion results in conversion of a substantial proportion of any quartz in the alumina process feedstock to sodium aluminosilicate, which deports to the digestion residue along with sodium aluminosilicate formed from more reactive forms of silica. Quartz is not significantly digested in low temperature digestion. Alumina process feedstocks having high contents of monohydrate alumina will, for an equivalent quartz and total silica content, consume more caustic soda, requiring greater make up of this expensive chemical. Further, such feedstocks will normally therefore benefit from treatment for the removal of liberated quartz particles prior to supply to the alumina refining process, at a further cost and process complexity, and usually for considerable loss of mineral values.
Another influence of high temperature digestion is the conversion of some iron in the alumina process feedstock to soluble and colloidal forms which are able to pass through the clarifying system and deport in large measure to the precipitated alumina hydrate. The iron content of alumina hydrate, along with the silica content, is an important determinant of the value of the calcined hydrate to aluminium smelter customers, as it affects the quality of high purity metal which can be made. The combination of high iron in clarified liquors (driven by monohydrate alumina in the alumina process feedstock) with low alumina yield in precipitation of alumina hydrate (driven as indicated above by organic impurities in the alumina process feedstock as well as the monohydrate alumina in the alumina process feedstock) is potentially very damaging for product quality, especially when combined with the implications for silica in hydrate of a sweetening process.
It will be apparent from the above discussion of the Bayer alumina refining process that there are two properties of an alumina process feedstock which have the dominant influence on complexity, and productivity in the Bayer process to which it is fed, as well as a significantly negative influence on hydrate product quality and a further negative influence on construction and operating costs, especially consumables costs. The first is the monohydrate alumina content, and the second is the content of extractable organic carbon (including oxalate precursor organics and hydrate seed poisons).
With the exception of processes involving high temperature reaction of the alumina process feedstock with or without reagents at high temperatures (see below) prior art processes for dealing in part with the latter of these problems, namely extractable organic carbon, are universally dependent on the treatment of a side stream of caustic liquors in the Bayer process for removal and destruction of compounds derived from the organic inputs. In one prior art process a side stream of caustic liquor is evaporated and mixed with a stream of alumina bearing dust and recycled solid calcined material before being fed to a high temperature calcination process in which all organic matter is destroyed by pyrolysis and combustion processes. The solid calcined product, consisting primarily of sodium aluminate, is divided into product and recycle components. The product component is either recycled into the Bayer process for dissolution, thereby recovering alumina and soda components, or used for dissolution for the production of specialty alumina hydrate products.
In another prior art process pressurised industrial oxygen is injected into circulating high temperature digestion liquors (possibly as a side stream, but also possibly in the main stream) to have the effect of conversion of organic impurities to oxidised gaseous species, and dissolved sodium carbonate, simpler organic compounds, and sodium oxalate. This process is always coupled with side stream processes for the removal of products of pressure oxidation, such as by causticisation with lime for the removal of carbonate, and side stream xe2x80x9csalting out evaporationxe2x80x9d in which a side stream is evaporated essentially to a cake of sodium salts including aluminate, carbonate, oxalate and organic compounds. This cake is either disposed of, or subjected to thermal decomposition for recovery of sodium and alumina values.
Oxalate removal from the circuit is also conducted on a side stream, either the fine seed wash liquors or a stream of solid oxalate made by crystallisation from an evaporated side stream of spent liquor. The oxalate is reacted with lime to produce a calcium oxalate precipitate which is disposed of with red mud or, in the case of solid oxalate, can be thermally decomposed, usually in a process for destruction of other organics contained in concentrated liquors.
Removal of carbonate by reaction with lime is also conducted on a side stream, in this case the wash liquors from solid residue washing.
The difficulty with side stream processing for the removal of organics and their derivatives such as carbonate and oxalate is that side stream processing can only be effective if these impurities have already reached a high level, usually already having a significant nuisance value, in the main liquor circuit through digestion and precipitation. The effectiveness of these processes in purifying liquors is limited because an enduring problem must already exist for these processes to be effective in reducing what would otherwise be a larger problem.
A process involving thermal treatment of a predominantly trihydrate alumina process feedstock at sufficient temperatures to result in partial elimination of organic carbon by pyrolysis and thermal oxidation has been described by Rijkeboer, along with a literature review of the art. In this process trihydrate alumina is dehydrated and the level of organic material which is extractable in. caustic solutions is significantly reduced. Specifically referred to are the patents of Kobayashi and Brown. Each of these prior art documents disclose that such thermal treatments, if properly applied, can result in no loss of alumina extractability compared to the original gibbsitic bauxite. Kobayashi indicates that success lies in maintaining a molar ratio of bound water to alumina (Al2O3) below 0.5. Brown specifically requires temperatures to be maintained in the range 300C. to 400xc2x0 C. for 10 to 120 minutes. Rijkeboer demonstrates that even with a test for extraction which provides for an optimistic view of extraction in the Bayer process (since it commences with pure caustic soda liquors instead of simulated spent Bayer liquor) the conditions indicated by Brown result in loss of extraction in realistic thermal processing equipment through the conversion of trihydrate alumina in feed to monohydrate alumina in the form of boehmite. Rijkeboer recommends a final temperature range of 400 to 600xc2x0 C. and a retained chemical water below that of Kobayashi""s limitation. He also indicates that a limitation to the process if extractability is not to be adversely affected is that the highest temperature treatment should be conducted at water vapour pressures of less than 2 kPa. This limitation is extreme from an industrial processing point of view, since most industrial fuels will upon combustion to introduce sufficient heat for dehydration at the required temperature produce water vapour levels in combustion gases in excess of 2 kPa. Therefore the only means of conducting the process would be by heat transfer via heating elements which are themselves heated either electrically or via the combustion of fuel. For industrial processes treating at least hundreds of thousands of tonnes (and most probably millions of tonnes) of feed per year the required heat transfer area (of the heating elements) will not result in an economically attractive outcome. Further, the water vapour pressure associated with completion of dehydration of the feed will be higher than 2 kPa unless there is very high dilution with air or some other gas, which even should heating via heating elements be used would result in the generation of large quantities of hot gases from which heat recovery in preheating and drying the feed would not be practical. Consequently none of the thermal processes proposed in the prior art which would have the impact of removal of organic matter accompanied by thermal dehydration while not significantly affecting the extractability of alumina from alumina process feeds can be operated under industrially realistic conditions.
There is also prior art reference (Russell, 1955) to the extraction of monohydrate alumina in the form of boehmite by heating boehmite in air to lower water content forms of hydrated alumina, conducted in such a manner that the product could be dissolved to a greater extent in hot caustic solutions than the original monohydrate alumina. However, since most alumina process feedstocks contain both monohydrate and trihydrate forms, and this prior art did not include conditions for the simultaneous dehydration of monohydrate and trihydrate forms which would not affect the properties of the trihydrate decomposition product, and there was no attempt to ensure that a significant water vapour pressure was present, the disclosure did not in any way overcome the problem identified by Rijkeboer of water vapour sensitivity. This disclosure did not therefore indicate an industrially realistic means of dealing with monohydrate in alumina process feeds, or of removing organic compounds under such industrially realistic conditions.
There is no known industrially realistic prior art process which presents a solution to the problems caused by monohydrate alumina in alumina process feedstocks save for processes which react the alumina process feed with other chemical reagents including soda (or soda ash) and lime (or limestone) at high temperatures. These processes are generally applied to alumina process feeds having high contents of silica which would digest and consume soda as sodium aluminosilicates in the Bayer process operated without this additional step. The processes produce calcium silicates (as by-products) in place of sodium aluminosilicates, and virtually all of the hydrated alumina (both trihydrate and monohydrate) in feed is converted to sodium aluminate. For alumina process feeds containing up to about 10% silica it is generally more economic to apply the Bayer process. That is, for most alumina process feeds these processes come with a significant economic penalty, in capital costs and in energy consumption.
The need for an industrially realistic process which can significantly improve an alumina process feedstock containing both organic carbon and monohydrate alumina so that the many negative implications of these characteristics for alumina refinery complexity and capital costs has been clearly recognised in the prior art. Virtually all processes which have been proposed to meet this need are deficient, either in not completely resolving the alumina refining difficulties, or in coming with a net economic penalty, or in adding net alumina refining complexity, or in being impractical for realistic industrial application in an alumina refining context.
A process which is effective in meeting this need has been identified, but without any of the above deficiencies. It has been surprisingly found that this process is most effectively applied under a particular range of conditions. These conditions would normally limit the thermal efficiency of the identified process, providing a significant economic disadvantage in energy consumption and equipment costs.
Accordingly, the present invention provides improvements in a process for the treatment of an alumina process feedstock for the simultaneous enhancement of achievable alumina digestion per unit of spent liquor and reduction in extractable organic carbon, which improvements optimise process effectiveness or reduce the loss of thermal efficiency without otherwise compromising the effectiveness of the process.